Industrial processes requiring production of steam or other high temperature process fluids are subject to equipment fouling and scale formation issues. An example of one such process is the production of high-quality steam for SAGD (steam assisted gravity drainage) in the recovery of bitumen. The affected equipment may include, for example, water treatment operations, steam boilers, and once through steam generators (OTSG).
Deposition and scaling at heat exchange surfaces occurs because temperature, concentration, and pressure changes disrupt solubility equilibria to cause solids formation. Deposited substances are largely combinations of inorganic cations and inorganic and organic anions. The primary cations for scale formation are ions of Ca, Mg, Fe, and Mn. These cationic species combine with anionic species including SiO2, CO32−, Cl−, and organic acids (humic and naphthalenic). Other elements that may contribute to fouling are Cu, Al, Na, Ba, Sr, K, Rb, Cs, and Li. Boiler fouling and scale formation may lead to significant costs due to losses in steam production efficiency and costly down-time. In spite of the importance of dissolved inorganic ions to boiler integrity, there is currently no on-line means of monitoring these metal ions at relevant concentrations for real-time process control.
Simultaneous multi-element analysis of metal ions is normally performed by lab-based techniques, such as inductively coupled plasma atomic emission spectrometry (ICP-AES). ICP-AES has never been adapted to on-line measurements because of high argon gas consumption and the requirement of frequent recalibrations due to instrument drift. However, a novel plasma spectrochemical technique has been described that does not consume inert gas and avoids instrument drift issues that plague traditional techniques. This technique is called solution cathode glow discharge (SCGD) and has shown linear calibration with detection limits in the low parts per billion range (see: Greda, K., et al., Comparison of performance of direct current atmospheric pressure glow microdischarges operated between a small sized flowing liquid cathode and a miniature argon or helium flow microjets, J. Anal. At. Spectrom., 28, 1233-1241 (2013) and Doroski, T. A., et al., Solution-cathode glow discharge-optical emission spectrometry of a new design and using a compact spectrograph. J. Anal. At. Spectrom., 2013. 28: p. 1090-1095). SCGD appears to be an ideal technique for simultaneous multi-element analysis of metal ions in on-line applications.
From the academic literature, a representation of the solution cathode glow discharge is shown in FIG. 1. This design has superior analytical performance and simplicity compared to previous published versions. The glass capillary extends 3 mm above the grounded graphite rod and the tungsten anode is 3 mm above the glass capillary. Electrical contact between the tip of the glass capillary and the graphite rod is made along the 3 mm vertical glass capillary by the liquid overflow of the solution cathode. Optimized electrical contact between the tip of the glass capillary and the graphite rod is made when the distance that the glass capillary extends above the graphite is minimized. However, distances less than 3 mm promote a glow-to-arc transition where the plasma anchors to the graphite rod as opposed to the tip of the glass capillary. Electrical arcing can destroy electrode components and prohibits the analytical performance of the instrument. Therefore, a compromised distance of 3 mm is used and 2.0 mL/min is the lowest sample flow rate that can be used before analytical performance degrades (see: Wang, Z., et al., Design modifications of a solution cathode glow discharge atomic emission spectrometer for the determination of trace metals in titanium dioxide. J. Anal. At. Spectrom., 2014. 00: p. 1-9 and Zhang, Z., et al., Determination of trace heavy metals in environmental and biological samples by solution cathode glow discharge atomic emission spectrometry and addition of ionic surfactants for improved sensitivity. Talanta, 2014. 119: p. 613-619). Lower flow rates degrade the analytical performance since the electrical connection through the fluid along the 3 mm glass capillary is degraded as flow rates decrease.
Within the patent literature, several variations of SCGD devices are disclosed. One of the earlier patents describing a SCGD device is U.S. Pat. No. 5,760,897 from Cserfalvi et al.; however, the inventors do not provide a proposed flow rate. Later patent application published WO/2007/012904, also from Cserfalvi et al. discloses a continuous flow rate of approximately 5-10 mL/min. China patent application CN 103163116 discloses the lowest flow rate achieved as 2.5 mL/min. U.S. Pat. No. 7,929,138 to Webb et al. discloses an SCGD configuration that facilitates analysis at low sample solution flow rates ranging from 2.0 to 3.0 mL/min. Although the inventors note that lower flow rates such as 1.5 mL/min. are also supported by the system, they disclose that their present method enables analysis between 2.0 and 2.5 mL/min. The flow rates in the Webb system are limited by the distance between the base of the plasma and the overflow solution in the reservoir in contact with the grounding electrode, which creates a greater resistance. There is therefore a need for an SCGD apparatus capable of flow rates below 2.0 mL/min that maintains a stable plasma emission and does not degrade the analytical performance.
To initiate the plasma in an SCGD device, a spark is required to jump the gap between the anode and flowing solution cathode and in the past this has been accomplished by one of two methods. Currently, the most common method is to physically lower the anode until it is within 1 mm of the cathode and then apply power from the dc power supply. At less than 1 mm distance, common dc power supplies have a sufficient voltage limit to jump the gap and initiate the plasma. Once the plasma is lit, the anode can be retracted to leave a 3 mm gap between electrodes. Thus, this method requires a mechanical mechanism to move the anode up and down, which has potential for wear and breakage. If the anode could be fixed in position, a simpler and more robust anode/cathode configuration can be built. Another method to initiate the plasma is to add a second high voltage power supply where the voltage, in excess of 10,000 V, is used solely to initiate the plasma by jumping the 3 mm gap between electrodes. This method runs the risk of damaging the main power supply that drives the plasma. There is therefore a need in the art for a method of initiating the plasma in SCGD that allows for a fixed configuration of the anode and cathode and does not require a second power supply.
To date, SCGD devices have primarily been used for the analysis of metal ions in aqueous solutions. Molecular emissions have been seen as background but SCGD devices have not been previously used for analysis of molecular species. Oxides, nitrides, and hydrides are classes of molecular species that can be formed in atmospheric pressure plasmas and can potentially be detected by molecular emission.
Isotopic analysis is an essential technique in the fields of medicine, chemistry, materials science, archeology, hydrology, carbon dating, and nuclear forensics. Traditionally, isotopic information has been determined by sophisticated isotope ratio mass spectrometers. Recently, laser ablation molecular isotopic spectrometer (LAMIS) has been used to provide isotopic analysis based on optical emission of molecular species. LAMIS has been shown to measure isotopes of hydrogen, boron, carbon, nitrogen, oxygen and chlorine (see: Bol'shakov, A. A., et al., Laser ablation molecular isotopic spectrometry for rare isotopes of the light elements. Spectroscopy, 2014. 29(6): p. 30-39). Although SCGD has not been previously disclosed for isotope measurement, isotopic analysis can be more practically accomplished using molecular spectra since the difference in isotopic masses has only a small effect on the electronic transitions in atoms, but a relatively large effect on the vibrational and rotational energy levels in molecules.
It is, therefore, desirable to provide improved apparatus and methods for SCGD.